The news lately is replete with mentions of sea ice, because NOAA’s Arctic Report Card features it prominently, mentioning that “In 2018 Arctic sea ice remained younger, thinner, and covered less area than in the past. The 12 lowest extents in the satellite record have occurred in the last 12 years.” It also emphasizes how the loss of Arctic sea ice is but the start of a chain of events leading to dramatic change both in the Arctic and elsewhere.
For those of us who keep track of Arctic sea ice, this is a story we know well. Of course some news reports have treated it all as a stunning revelation, but the NOAA report is highlighting trends that we’ve known about for years, decades even. Rather interesting is what they have to say about the changes of ice age:
Older ice tends to be thicker and is thus more resilient to changes in atmospheric and oceanic heat content compared to younger, thinner ice. The oldest ice (> 4 years old) continues to make up a small fraction of the Arctic ice pack in March, when the sea ice extent has been at its maximum in most years of the satellite record. In 1985, the oldest ice comprised 16% of the ice pack (Fig. 3a), whereas in March of 2018 old ice only constituted 0.9% of the ice pack (Fig.3b). Therefore, the oldest ice extent declined from 2.54 million km2 in March 1985 to 0.13 million km2 in March 2018, representing a 95% reduction.
The decrease from 2.54 to 0.13 is significant in every sense of the word.
I’d like to look at how Arctic sea ice has changed by season, but I’m going to re-define the seasons. The reason we usually think of summer as being the months Jun-Jul-Aug is that those are the three hottest months of the year (for us northern-hemisphere people). Let’s define a “sea ice summer/fall” as the three months with lowest sea ice extent. We can figure that out from a plot of sea ice against month of the year (rather than time). I’ll make a boxplot by month, which gives this:
We can see that the three lowest-extent months are Aug-Sep-Oct, so we’ll call that our “sea ice summer/autumn.” Similarly, we’ll define autumn/winter as Nov-Dec-Jan, winter-spring as Feb-Mar-Apr, and spring/summer as May-Jun-Jul. Now I can compute the yearly average sea ice extent for each season separately.
I wasn’t exaggerating when I mentioned our knowing for decades about sea ice trends. Let’s begin by looking for trends using only the data prior to the year 2000. The changing extents (transformed into anomalies, and with a separate line for each season) look like this:
First, they’re all trending downward, and all trends turn out to be statistically significant. We knew by 2000 (even before that, in fact) that Arctic sea ice was in decline during every season. Second, the summer/autumn extent, during the season of least sea ice, shows a lot more variation than the other seasons. Its trend isn’t declining any faster, but the size of the fluctuations is considerably greater.
Let’s re-draw that same graph, but with some extra room added, and I’ll put in the trend lines for each season, extended from year 2000 up to the present:
And what has happened since year 2000, you wonder? This:
First, the already-significant downward trends didn’t just continue, they got more severe. Second, the summer/autumn extent continued to show a lot more variation than the other seasons.
The most important information about sea ice in the Arctic Report Card is that its changes really are just the beginning of a chain of events, a cascade which has already had unpleasant consequences. It’s beyond our ken to fathom all of the future consequences, but some of the possibilities are truly frightening (e.g. permafrost melt).
Meanwhile at the global warming denier blog WUWT, Eric Worral can twist a NASA report from “increases in the rate at which Arctic sea ice grows in the winter may have partially slowed down the decline of the Arctic sea ice cover” into “Global Warming Promotes Arctic Sea Ice Growth.” What a rube.
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